This application claims Paris Convention priority of DE 199 17 632 filed Apr. 19, 1999 the complete disclosure of which is hereby incorporated by reference.
Conventional opto-electronic sensor systems for the analysis of substances, e.g. using absorption or fluorescence measurements, have the decisive advantages of selectivity, high sensitivity and the possibility of rapid detection of the respective types and concentrations of substances of interest in liquid, gaseous and solid substance mixtures. Linked with these decisive advantages, however, is the disadvantage that important optical components become soiled, such as e.g. optical measuring cells, optical windows, optical radiation sources and optical sensors. This can lead to measurement errors or total failure of the measurement. Prior art discloses various methods and devices, such as e.g. the multi-wavelength method using one or more reference wavelengths according to the patents P 40 30 959, P 40 30 960, EP 91 102 499, and DE 198 494 847. To compensate for the soiling effect, radiation of different wavelengths is transmitted through the substance mixtures and radiation emitted directly by the radiation sources, at the respective time, is detected by a reference sensor and taken into consideration for evaluating the signal (e.g. see the patents DE 36 15 259, P 41 38 419, EP 0383 072).
All of these methods meet practical limitations, since so-called non-gray soiling leads to spectral disturbances which cannot be completely eliminated by the conventional compensation methods and since severe soiling, which occurs in practice, can weaken the light intensities of the various beams to such an extent that the residual light is no longer sufficient for the measurement. This occurs e.g. unavoidably with measurement of isopropanol or isopropanol additive mixtures in the fountain solution of offset printing machines, since the salt content of the paper to be printed, suspended matter from the paper, foreign substances from the paint, chemical additives etc. contaminate the liquid to be measured. This falsifies the optical measurement and eventually prevents carrying it out.
Corresponding effects also occur if one tries to measure the substances of interest in the gas located above the liquid phase, i.e. in the so-called xe2x80x9cheadspacexe2x80x9d, since the aerosols generated by the motion of the fountain solution also contain the above-mentioned contaminating and disturbing substances which deposit on the surfaces of the optical components, even when fine pore filters are used.
It is therefore the underlying purpose of the present invention to automatically eliminate the physically unavoidable deposit of soiling or disturbing substances on optical components of sensor systems for the quantitative measurement of the concentrations of the substances contained in liquids or gases and solids, to guarantee correct and interference-free measurement at all times. This methodical measurement is difficult since, in particular, the processing technology must not be disturbed. In particular, there must be no noticeable interruption in the measurement and subsequent dosing processes e.g. of isopropanol or additives. Furthermore, effects due to physically unavoidable residual soiling, which can vary with time depending on the type of soiling, must not distort the measurements.
Up to this point in time, this difficult object could be solved e.g. neither in offset printing nor in many other practical applications, due to 100% automation requirements, i.e. without any manual operation. The present invention achieves this goal with the cyclic use of cleaning substances and combined application of reference liquids. The invention also provides for timed triggering of the above-mentioned procedures, controlled by the opto-electronic sensor system for substance analysis and with the control of valves for activating the cleaning and reference liquid circuits. These circuits are hermetically sealed with respect to one another and from the process circuit, e.g. the fountain solution circuit of an offset printing machine.
Individual embodiments are described in more detail below for illustration of the present invention.